Switching regulators are used to control the flow of power from an input voltage supply to a load.
In general, a switching regulator includes a minimum of two switching elements and energy storing/filtering components as well as control circuitry. The control circuitry turns the switches on and off to provide constant output voltage with variable input voltage and load conditions. The energy storing/filtering components usually comprise an inductor and a capacitor, but can also include a transformer. Of the two switching elements, in most cases, only one has to be controlled, the other can be a diode. (The only exception is when bidirectional energy transfer between the input and output is used).
During the controlled switch's on-time, energy is stored into the inductor, while the diode is reverse biased, separating the output from the input. When the switch is turned off by the control circuitry, the diode turns on, transferring all or part of the stored energy to the output. If only part of the energy is transferred ("continuous" operating mode), then the diode is still forward biased (conducting current) when the switch turns on. To turn off the diode, the stored charge in its junction has to be depleted. During this time period (reverse recovery time), the diode behaves like a short circuit and draws a large transient current spike from the input through the controlled switch. If, as in most cases, the control circuitry includes a subcircuit that limits or controls the peak switch current on a cycle by cycle basis, this transient current glitch can trigger the current limit circuitry causing the instantaneous turn-off of the power switch. This malfunction prevents the switching regulator from operating correctly and from transferring any significant amount of power from the input to the output.
In the past, two conventional approaches have been employed to protect the current limit comparator during the reverse recovery time of the diode. One approach uses a low pass filter (e.g., an RC filter) to filter out the current spikes. Alternatively, the current comparator may be disabled (blanked) for a fixed amount of time after it is turned on. With either approach, the circuit has to be strongly overdesigned to accommodate for the worst case scenario for a variety of different diodes.
It would therefore be desirable to have available a switching regulator wherein the current limit sense/comparator circuitry is disabled adaptively only during the diode's reverse recovery time.